When fabricating this type of device, contacts are formed that have a resistance that is as low as possible in order to obtain uniform distribution of electric current in the active part of the device.
In particular, such devices are fabricated on a thick, highly doped GaN layer in order to obtain the lowest possible sheet resistance, preferably less than 5Ω (ohm). Sheet resistance is defined as the ratio between electric resistivity and film thickness. The impact of sheet resistance on the uniformity of current in a planar LED structure is described in the article by X. Guo et al. published in Appl. Phys. Lett. 78, 21:3337 (2001).
A structure of a known compound is schematically illustrated in FIG. 1. It comprises an assembly of layers forming the device, e.g., stack 5, resting on a substrate 7. Contacts 81, 82 are respectively applied on the upper layer of the stack 5 and on the lower layer thereof, respectively.
U.S. Patent Application Publication No. 2006/0211159 describes a technique for fabricating an LED with electrical contacts on a metallic contact layer, which allows the required thickness of doped semiconductor material to be reduced, while providing contact with low resistance.
This technique is schematically illustrated in FIGS. 2A-2E.
A composite substrate comprising a GaN growth layer 12 bonded on a mechanical carrier 2 in sapphire (FIGS. 2A and 2B) is formed by assembly of the carrier 2 with a substrate 1 in GaN. The latter is previously implanted to form a weakened region 17 therein, separating it into two parts: the layer to be transferred 12 and the negative layer 11 of the substrate 1.
An epitaxy of layers 5 used to form the device, is then conducted on the growth layer 12 (FIG. 2B).
A metallic layer 6 is deposited on this stack, after which a substrate 7 adapted to the functioning of the LED is bonded onto the layer 6 (FIG. 2C) before detaching the growth carrier 2. This latter operation is performed by means of laser radiation 10 directed through the transparent carrier 2, toward the interface between the carrier 2 and the GaN layer 12 (FIG. 2D). In this manner, decomposition of an interface layer is achieved, not illustrated here, via thermal effect resulting from the action of the laser beam allowing the carrier 2 to be detached from the remainder of the structure.
It is then possible to etch the stack remaining on the substrate 7 down to the layer 6 and to form contact regions 81, 81′, 81″, 82, 82′, 82″, first, on this metallic layer 6 and second, on the portions of the layer 12 in GaN (FIG. 2E). This gives rise to an assembly of independent, individual devices that can be separated along the detachment lines shown in FIG. 2E as vertical dashed lines.
It is thus possible to form contacts on the portions of metallic layer 6 that are located between the stacks 5.
This technique requires the transferring of epitaxy structure 5 onto a final substrate 7 that, in addition to a bonding step, involves a step to turn over the structure (which may cause damage thereto), followed by a step to detach the growth carrier 2 by laser radiation 10.
The problem, therefore, arises in finding another method for forming a structure that, in addition to an assembly of layers forming an active planar component, comprises a substrate carrying this component and contact regions for the functioning of this structure.